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Petrophysics of Bimodal Porosity: Lower Cretaceous Rodessa Limestone, Running Duke Field, Houston County, Texas
G. B. Asquith, A. D. Jacka
The Running Duke field in Houston County, Texas is developed on an east-west oriented domal structure with production from the Lower Cretaceous Rodessa Limestone. The reservoir rocks consist of ooid grainstone, ooid skeletal grainstone and skeletal grainstone.
Micritization of the ooids by early fresh water diagenesis has created intragranular microporosity. The ooid grainstone reservoir, therefore has a bimodal pore system which consists of intragranular microporosity and intergranular megaporosity. The presence of the bimodal pore system in the ooid grainstone results in high water saturations (55 to 87 percent), because the intragranular microporosity is water-filled.
In order to determine the true productive potential of these bimodal reservoirs the effective water saturation in the intergranular megaporosity and the amount of effective intergranular megaporosity was determined by combining mercury injection and petrographic analysis with log analysis. First the percentages of both the micro- and megaporosities were determined using mercury injection and petrographic analysis. Then total water saturation was corrected to effective water saturation. Uncorrected total water saturations for the ooid grainstones averaged 69.4 percent. The corrected effective water saturations averaged 34.9 percent. The much lower effective water saturations calculated by a combination of log analysis plus mercury injection and/or petrographic analysis indicates the true productive potential of these bimodal reservoirs.
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